Wide belt sander

ABSTRACT

A wide belt sander has a base, a driving axle, a driven axle bracket, a driven axle, a belt and an automatic adjustment device. The driving axle has a central axis. The driven axle bracket is pivotally mounted on the base along a longitudinal axis perpendicular to the central axis of the driving axle. The driven axle is rotatably mounted on the driven axle bracket along a rotating axis parallel with the central axis of the driving axle. The automatic adjustment device drives the driven axle bracket to swing relative to the base along the longitudinal axis in a simple harmonic motion and has two pushed elements and a cam. The pushed elements are mounted separately on the driven axle bracket. The cam is mounted between and alternatively pushing the pushed elements to swing the driven axle bracket relative to the base.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sander, and more particularly to awide belt sander having adjustment devices to make a belt movingsmoothly and keep deviations from occurring.

2. Description of Related Art

To polish surfaces of an article, a sander is always used andsubstantially comprises a base, two axles and a belt. The axles arerotatably mounted on the base and are arranged in parallel, and the beltis endlessly mounted around the axles and is driven to polish anarticle. To keep the belt from deviating during the polishing process,an adjustment device is always arranged on the conventional sander. Forexample, Taiwan Patent No. 1279285, entitled to “Wide Belt Sander With AReciprocal Adjustment Device” discloses an adjustment device having twocams mounted respectively at two sides of a pushed element that issecured on a driven axle bracket. When the rotation of the cams, thepushed element is alternatively pushed by the cams to make the drivenaxle bracket to swing in a simple harmonic motion. Accordingly, thetravel track of the running belt can be automatically and activelyadjusted to prevent deviation of the belt from occurring.

However, the conventional adjustment device has the following drawbacks.

1. The cams have to be rotated simultaneously but in reverse, a specificgear box is necessary. The gear box of the conventional adjustmentdevice has to be designed or manufactured specifically, so the cost formanufacturing is high.

2. The belt of the conventional sander still deviates due to unevenforce applied to the running belt during the polishing process even whenthe conventional adjustment device is arranged.

To overcome the shortcomings, the present invention tends to provide awide belt sander to mitigate or obviate the aforementioned problems.

SUMMARY OF THE INVENTION

The main objective of the invention is to provide a wide belt sanderhaving an adjustment device to make a belt moving smoothly and keepdeviations from occurring.

The wide belt sander in accordance with the present invention has abase, a driving axle, a driven axle bracket, a driven axle, a belt andan automatic adjustment device. The driving axle is rotatably mounted onthe base and has a central axis. The driven axle bracket is pivotallymounted on the base along a longitudinal axis perpendicular to thecentral axis of the driving axle. The driven axle is rotatably mountedon the driven axle bracket along a rotating axis parallel with thecentral axis of the driving axle. The belt is endlessly mounted aroundthe driving and driven axles. The automatic adjustment device is mountedbetween the base and the driven axle bracket to drive the driven axlebracket to swing relative to the base along the longitudinal axis in asimple harmonic motion and has two pushed elements and a cam. The pushedelements are mounted separately on the driven axle bracket. The cam isrotatably mounted on the base and is mounted between and alternativelypushing the pushed elements to swing the driven axle bracket relative tothe base.

Other objects, advantages and novel features of the invention willbecome more apparent from the following detailed description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wide belt sander in accordance withthe present invention;

FIG. 2 is a partially exploded perspective view of the wide belt sanderin

FIG. 1;

FIG. 3 is an enlarged perspective view of a contacting switch of thewide belt sander in FIG. 1;

FIG. 4 is an enlarged front view of the wide belt sander in FIG. 1;

FIG. 5 is an enlarged top view in partial section of the gear box of thewide belt sander in FIG. 1;

FIG. 6 is an enlarged operational side view in partial section of thegear box in FIG. 5;

FIG. 7 is an enlarged operational top view of the automatic adjustmentdevice of the wide belt sander in FIG. 1 showing that the cam is at the0° position;

FIG. 8 is an enlarged operational top view of the automatic adjustmentdevice in FIG. 7 showing that the cam is at the 90° position;

FIG. 9 is an enlarged operational top view of the automatic adjustmentdevice in FIG. 7 showing that the cam is at the 180° position;

FIG. 10 is an enlarged operational top view of the automatic adjustmentdevice in FIG. 7 showing that the cam is at the 270° position;

FIG. 11 is an enlarged perspective view of the electrical adjustmentdevice of the wide belt sander in FIG. 1; and

FIG. 12 is an enlarged operational end view in partial section of thewide belt sander in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIGS. 1, 2 and 4, a wide belt sander in accordancewith the present invention comprises a base 10, a driving axle 20, adriven axle bracket 30, a driven axle 40, a belt 50, an automaticadjustment device and an electrical adjustment device.

The base 10 comprises a bottom frame, a column 12, a slider 13, a motor14, a holding bracket 15 and an axle frame 150. A second belt 11 may beendlessly mounted on the bottom frame in a horizontal direction toprovide a further polishing effect. The column 12 is upwardly mounted onthe bottom frame. The slider 13 is slidably mounted on the column 12with a threaded rod that is mounted rotatably on the column 12. Adove-tail engagement is mounted between the slider 13 and the column 12to allow the slider 13 slidable relative to the column 12 smoothly. Themotor 14 is mounted on the slider 13. The holding bracket 15 is securelymounted on the slider 13 above over the bottom frame and has a top, anaxle hole 151, a channel 152 and two threaded holes. The axle hole 151is defined in the top at the middle of the holding bracket 151. Thechannel 152 is defined in the top near one end of the holding bracket15. The threaded holes are defined in the top of the holding bracket 15near the channel 152. The axle frame 150 is securely attached to theholding bracket 15 at one end opposite to the slider 13.

The driving axle 20 is rotatably mounted between the slider 13 and theaxle frame 150 below the holding bracket 15 and has a central axis. Thedriving axle 20 is connected to and driven by the motor 14. In addition,the channel 152 in the top of the holding bracket 15 is defined along adirection perpendicular to the central axis of the driving axle 20.

The driven axle bracket 30 is pivotally mounted on the base 10 along alongitudinal axis perpendicular to the central axis of the driving axle20. The driven axle bracket 30 has a central shaft 31 formed on andprotruding downward from the bottom of the driven axle bracket 30 androtatably and slidably extending into the axle hole 151 in the holdingbracket 15.

In addition, a pneumatic cylinder 60 is mounted between the holdingbracket 15 and the driven axle bracket 30. The pneumatic cylinder 60 hasa first end connected to the driven axle bracket 30 and a second endconnected to the holding bracket 15 with a universal jointer. Thecylinder 60 can provide a supporting effect to the driven axle bracket30 to keep the driven axle bracket 30 away from the driving axle 20 at adesired distance.

The driven axle 40 is rotatably mounted on the driven axle bracket 30along a rotating axis parallel with the central axis of the driving axle20. The belt 50 is endlessly mounted around the driving and driven axles20,40.

Additionally, an axle adjusting device is mounted between the base 10and the driven axle bracket 30 to change a distance between the drivingand driven axles 20,40. The axle adjusting device comprises a handlebar61 and a pushing plate 64. The handlebar 61 is pivotally mounted on theholding bracket 15 with a pivot 62 and has a guiding channel definedthrough the handlebar 61 at one end of the handlebar 61. The pushingplate 64 is pivotally mounted on the holding bracket 15, is pivotallyand slidably connected to the guiding channel in the handlebar 61 with asliding pivot 63 and is connected to the central shaft 31. When thehandlebar 61 is pulled upward, the driven axle bracket 30 is movedtoward the driving axle 20 with the transmission of the pushing plate 64and compresses the cylinder 60. Accordingly, the distance between thedriving and driven axles 20,40 can be shortened, and to replace a wornbelt 50 with a new one is convenient.

When the handlebar 61 is released, the driven axle bracket 30 can bepushed upward to the original position where the belt 50 is at atightened condition for polishing with the recoil force provided by thepneumatic cylinder 60.

With reference to FIGS. 1, 2 and 4 to 6, the automatic adjustment deviceis mounted between the base 10 and the driven axle bracket 30 to drivethe driven axle bracket 30 to swing relative to the base 10 along thelongitudinal axis in a simple harmonic motion. The automatic adjustmentdevice comprises a gear box 70, a cam 71, two pushed elements 72,73 anda driving assembly. The gear box 70 is adjustably mounted on the holdingbracket 15 of the base 10. The gear box 70 has a bottom and two ribs 701formed on the bottom of the gear box 70 and slidably held in the channel152 in the holding bracket 15. A wing 702 is mounted on and laterallyextends from the bottom of the gear box 70 and has two elongated holes703 defined through the wing 702 and aligning respectively with thethreaded holes in the holding bracket 15. With two bolts 704 extendingthrough the elongated holes 703 in the wing 702 and screwed into thecorresponding threaded holes in the holding bracket 15, the gear box 70is securely mounted on the holding bracket 15. With releasing the bolts704, the gear box 70 can be moved along the channel 152 in the holdingbracket 15 to adjust the position of the gear box 70 relative to theholding bracket 15.

The gear box 70 comprises a driving shaft 705, a driven shaft 708 and agear device. The driving shaft 705 is rotatably mounted on and extendsinto a side of the gear box 70. The driven shaft 708 is rotatablymounted on and extends out from the top of the gear box 82. The geardevice is mounted in the gear box 70 and is mounted between the drivingshaft 705 and the driven shafts 708. The gear device comprises a wormrod 709 and a worm gear 709′. The worm rod 709 is co-axially mounted onthe driving shaft 705. The worm gear 709′ is securely mounted on thedriven shaft 708 and engages the worm rod 709. Accordingly, the drivenshaft 708 can be rotated with the driving shaft 705 at a reduced speed.

The cam 71 is mounted securely on and driven by the driven shaft 708.Preferably, the cam 71 is mounted on a middle position of the top of thegear box 70.

The pushed elements 72,73 are mounted on the bottom of the driven axlebracket 30 at an interval. The pushed elements 72,73 have a center linedefined between centers of the pushed elements 72,73. The cam 71 ismounted between the pushed elements 72,73 and alternatively pushes thepushed elements 72,73 to swing the driven axle bracket 30 relative tothe base 10 as shown in FIGS. 7 to 10. The cam 71 has a rotating centerwhere is mounted around the driven shaft 708 and located at a middlepoint of the center line between the pushed elements 72,73. Thus, thecam 71 has an equal distance to each pushed element 72,73. Preferably,each pushed element 72,73 is a bearing having a central hole 720,730defined through the pushed element 72,73. Two securing bolts 721,731 aremounted respectively through the central holes 720,730 of the pushedelements 72,73 and mounted securely to the bottom of the driven axlebracket 30 to mount the pushed elements 72,73 on the driven axle bracket30.

The driving assembly is mounted on the base 10 to drive the cam 71 torotate and comprises a driving pulley, a driven pulley 706 and a drivingbelt 707. The driving pulley is co-axially connected to and rotated withthe driving axle 20. The driven pulley 706 is co-axially connected tothe driving shaft 705. The driving belt 707 is mounted around thedriving pulley and the driven pulley 706. When the driving axle 20 isrotated by the motor 14, the cam 71 will also rotate with thetransmission of the driving assembly, the driving shaft 705, the geardevice in the gear box 70 and the driven shaft 708.

Additionally, an indicating board 80 is securely mounted between theholding bracket 15 and the driven axle bracket 30 and has multiplemounting holes 801, an upper indication channel 81 and a lowerindication channel 82. The mounting holes 801 are defined through theindicating board 80. Multiple bolts 802 are mounted respectively throughthe mounting holes 801 and screwed into the threaded holes 803 definedin the holding bracket 15 and the driven axle bracket 30 to secure theindicating board 80 on the holding bracket 15 and the driven axlebracket 30. The upper indication channel 81 and the lower indicationchannel 82 are laterally defined through the indicating board 80 and areparallel with each other. Each indication channel 81,82 has an indexmark 83 formed on an edge of the channel 81,82. An upper pointer 84 ismounted on the driven axle bracket 30 at the middle point of the centerline between the pushed elements 72,73 and extends into the upperindication channel 81 in the indicating board 80. A lower pointer 85 issecurely mounted on the gear box 70 at a position aligning with the cam71 and extends into the lower indication channel 82 in the indicatingboard 80. With the arrangement of the pointers 84,85 and the indicationchannels 81,82, the positions of the driven axle bracket 30 and the gearbox 70 are identified. When the driving and driven axles 20,40 arecompletely parallel with each other, the pointers 84,85 correspondrespectively to zero points of the index marks 83.

With reference to FIGS. 1 to 3, 7 and 11, the electrical adjustmentdevice is mounted on the holding bracket 15 of the base 10 to pivot thedriven axle bracket 30 relative to the base 10 so as to adjust a periodof the simple harmonic motion of the automatic adjustment device. Theelectrical adjustment device comprises two contacting switches 91,92,two electromagnetic switches 94,95, a holding frame 93, a connectingframe 96 and two springs 941,951. The contacting switches 91,92 aremounted on the top of the holding bracket 15 of the base 10 respectivelyat two sides of the belt 50. Each contacting switch 91,92 has acontacting arm 911 protruding from the contacting switch 91,92,selectively abutting with an edge of the belt 50 and having a ceramicsleeve 912 mounted around the contacting arm 911. The ceramic sleeve 912can keep the contacting arms 911 from being damaged or worn off by therunning belt 50.

The holding frame 93 is mounted securely on the top at one end of theholding bracket 15. The electromagnetic switches 94,95 are electricallyconnected respectively to the contacting switches 91,92 and are mountedon two ends of the holding frame 93 respectively at two sides of thedriven axle bracket 30. The electromagnetic switches 94,95 have a timingfunction to switch the electromagnetic switches 94,95 off automaticallyafter a predetermined time, for example 5 seconds.

The connecting frame 96 is mounted securely on one end of the drivenaxle bracket 30 and has a bottom having two sides and two ends and twoconnecting ears 962. The connecting ears 962 are respectively formed onand protrude from the two ends and the two sides of the bottom of theconnecting frame 96.

The springs 941,951 are connected respectively to the electromagneticswitches 94,95 and the driven axle bracket 30. Each spring 941,951 hastwo ends connected respectively a corresponding one of theelectromagnetic switches 94,95 and one of the connecting ears 962 on theconnecting frame 96. Accordingly, the springs 941,951 are connectedrespectively to the two sides of the driven axle bracket 30 via theconnecting frame 96 to provide pulling forces in opposite directions tothe driven axle bracket 30.

Before the wide belt sander is in used, the positions of the driven axlebracket 30 and the cam 71 of the automatic adjustment device must bezeroized. With reference to FIGS. 9 and 10, to zeroize the position ofthe driven axle bracket 30, the driving belt 707 is detached from thedriving and driving pulleys 706 and the motor 14 is switched on torotate the driving axle 20. At this time, the cam 71 is kept at a 0° or180° position as shown in FIGS. 7 and 9. With the rotation of thedriving axle 20, the driven axle 40 will be rotated with thetransmission of the belt 50 but the cam 71 keeps stationary. Because thedriven axle bracket 30 is pivotally mounted on the holding bracket 15with the central shaft 31, the angular position of the driven axlebracket 30 is adjusted manually relative to the holding bracket 15 tomake the belt 50 running in a desired travel track. After moving theindicating board 80 to make the zero point of the index mark 83 on theupper indication channel 81 aligning with the upper pointer 84, thebolts 802 are screwed completely into the threaded holes 803 in theholding bracket 15 and the driven axle bracket 30 to securely theindicating board 80 on the driven axle bracket 30 at the zeroized point.

Then, the bolts 704 for securing the gear box 70 are released, and theposition of the gear box 70 is adjusted along the channel 152 to makethe lower pointer 85 aligning with the zero point of the index mark 83on the lower indication channel 82. At this position, the cam 71 is atthe middle of the center line of the pushed elements 72,73. Accordingly,the driven axle bracket 30 and the cam 71 are zeroized, and the drivingbelt 707 is mounted around the driving and driven pulleys 706.

Consequently, the cam 71 can be rotated with the transmission of thedriving assembly, the driving shaft 705, the gear device and the drivenshaft 708 while the driving axle 20 is rotating. Accordingly, withreference to FIGS. 6 to 8, the pushed element 72,73 will bealternatively pushed by the cam 71 to make the driven axle bracket 30 toswing relative to the central shaft 31 in a simple harmonic motion. Withthe swinging of the driven axle bracket 30, the travel track of therunning belt 50 can be automatically and actively adjusted, such thatthe movement of the running belt 50 is stable and smooth. Within onerotation of the cam 71, the driven axle bracket 30 can be adjusted twiceand this means that two adjustment frequencies are provided. Themovement travel of the running belt 50 is smooth, and the belt 50 can bekept from being worn off at a single edge. The useful life of the belt50 can be prolonged and the belt 50 is durable.

If the sander is applied with an uneven force to cause the deviation ofthe belt 50, the belt 50 will abut against the contacting arm 911 of oneof the contacting switches 91,92. Thus, the pushed contacting switch91,92 will send a signal to activate the corresponding electromagneticswitch 94,95 to pull the corresponding spring 941,951 with a pullingforce of 0.8 to 1.5 Kg. Consequently, the pulled spring 941,951 willpull the driven axle bracket 30 to swing via the connecting frame 96,and the period of the simple harmonic motion of the automatic adjustmentdevice can be changed. Accordingly, the belt 50 can move backward thedesired travel track. After the predetermined time set up in theelectromagnetic switch 94,95, for example 5 seconds, the electromagneticswitch 94,95 is switched off and the driven axle bracket 30 can bepivoted to an original position with the recoil forces provided by thesprings 941,951. Therefore, another adjustment effect is provided, suchthat double adjustment functions can be provided by the automaticadjustment device and the electrical adjustment device.

With reference to FIGS. 2 and 12, a limiting device is mounted betweenthe driven axle bracket 30 and the holding bracket 15 to limit theswinging range of the driven axle bracket 30. The limiting devicecomprises a limiting bracket 97, two limiting bolts 972 and a basebracket 974. The limiting bracket 97 is mounted on the bottom of thedriven axel bracket 30, is inversed U-shaped and has two earsrespectively formed on and protruding from two ends of the limitingbracket 97. The limiting bolts 972 are mounted respectively on the earsof the limiting bracket 97 and protrude toward each other. The basebracket 974 is mounted securely on the top of the bolding bracket 15 andhas a top extending into the limiting bracket 97 to a position betweenthe limiting bolts 972.

Accordingly, when the driven axle bracket 30 is swung by the automaticadjustment device or the electrical adjustment device, the swingingrange of the driven axle bracket 30 can be limited by the abutment ofthe base bracket 974 with one of the limiting bolts 972.

With the aforementioned description, the wide belt sander has thefollowing advantages.

1. With a single cam 71 arrangement, the gear device in the gear box 70has not to be specifically designed and manufactured. The gear box 70can be a standardized product, such that the cost for manufacturing thewide belt sander can be reduced.

2. To assemble a single cam 71 onto the gear box 70 is easy andconvenient, and the zeroizing process of the cam 71 is also easy.

3. The wide belt sander in accordance with the present invention has twoadjustment means, such that the travel of the belt 50 can be wellcontrolled on a desired track. The useful life of the wide belt sandercan be prolonged and is durable.

Even though numerous characteristics and advantages of the presentinvention have been set forth in the foregoing description, togetherwith details of the structure and function of the invention, thedisclosure is illustrative only, and changes may be made in detail,especially in matters of shape, size, and arrangement of parts withinthe principles of the invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed.

What is claimed is:
 1. A wide belt sander comprising: a base; a drivingaxle rotatably mounted on the base and having a central axis; a drivenaxle bracket pivotally mounted on the base along a longitudinal axisperpendicular to the central axis of the driving axle; a driven axlerotatably mounted on the driven axle bracket along a rotating axisparallel with the central axis of the driving axle; a belt endlesslymounted around the driving and driven axles; and an automatic adjustmentdevice mounted between the base and the driven axle bracket to drive thedriven axle bracket to swing relative to the base along the longitudinalaxis in a simple harmonic motion and comprising two pushed elementsmounted separately on the driven axle bracket; and a cam rotatablymounted on the base and mounted between and alternatively pushing thepushed elements to swing the driven axle bracket relative to the base.2. The wide belt sander as claimed in claim 1 further comprising anelectrical adjustment device mounted on the base to pivot the drivenaxle bracket relative to the base to adjust a period of the simpleharmonic motion of the automatic adjustment device.
 3. The wide beltsander as claimed in claim 2, wherein the automatic adjustment devicecomprises a gear box adjustably mounted on the base 10; a driving shaftrotatably mounted on and extending into the gear box; a driven shaftrotatably mounted on and extending out from the gear box and on whichthe cam is mounted; and a gear device mounted in the gear box, mountedbetween the driving shaft and the driven shaft and comprising a worm rodco-axially mounted on the driving shaft; and a worm gear securelymounted on the driven shaft and engaging the worm rod.
 4. The wide beltsander as claimed in claim 3, wherein the pushed elements are rotatablymounted on a bottom of the driven axle bracket at an interval and have acenter line defined between centers of the pushed elements; and the camhas a rotating center where is mounted around the driven shaft andlocated at the center line between the pushed elements.
 5. The wide beltsander as claimed in claim 4, wherein each pushed element is a bearinghaving a central hole defined through the pushed element; and twosecuring bolts mounted respectively through the central holes of thepushed elements and mounted securely on the bottom of the driven axlebracket.
 6. The wide belt sander as claimed in claim 5, wherein the basehas a channel defined along a direction perpendicular to the centralaxis of the driving axle; and two threaded holes defined in the baseadjacent to the channel; and the gear box is mounted adjustably in thechannel in the base and has a bottom; two ribs formed on the bottom andslidably held in the channel in the base; a wing mounted on andlaterally protruding from the bottom of the gear box and having twoelongated holes defined through the wing and aligning respectively withthe threaded holes in the base; and two bolts mounted respectivelythrough the elongated holes and screwed respectively into the threadedholes in the base.
 7. The wide belt sander as claimed in claim 6,wherein the base further comprises a holding bracket to which the drivenaxle bracket is pivotally connected and the gear box is adjustablymounted and having an axle hole defined in a top of the holding bracket;the driven axle bracket has a central shaft formed on and protrudingdownward from the driven axle bracket and rotatably and slidablyextending into the axle hole in the holding bracket; and a pneumaticcylinder is mounted between the holding bracket and the driven axlebracket and has a first end provided with a universal jointer andconnected to the holding bracket with the universal jointer; and asecond end connected co-axially to the central shaft on the driven axlebracket to provide a supporting effect to the driven axle bracket. 8.The wide belt sander as claimed in claim 7, wherein the electricaladjustment device comprises two contacting switches mounted on the baserespectively at two sides of the belt; two electromagnetic switcheselectrically connected respectively to the contacting switches andmounted on the base respectively at two sides of the driven axlebracket; and two springs each having two ends connected respectively oneof the electromagnetic switches and the driven axle bracket, wherein thesprings are connected respectively to the two sides of the driven axlebracket to provide pulling forces in opposite directions to the drivenaxle bracket.
 9. The wide belt sander as claimed in claim 8, whereineach contacting switch has a contacting arm protruding from thecontacting switch selectively abutting with the belt and having aceramic sleeve mounted around the contacting arm.
 10. The wide beltsander as claimed in claim 9 further comprising a limiting devicemounted between the driven axle bracket and the holding bracket to limitthe swinging range of the driven axle bracket and comprising a limitingbracket mounted on the bottom of the driven axel bracket, being inversedU-shaped and having two ears respectively formed on and protruding fromtwo ends of the limiting bracket; two limiting bolts mountedrespectively on the ears of the limiting bracket and protruding towardeach other; and a base bracket mounted securely on the top of thebolding bracket and having a top extending into the limiting bracket toa position between the limiting bolts.
 11. The wide belt sander asclaimed in claim 2, wherein the electrical adjustment device comprisestwo contacting switches mounted on the base respectively at two sides ofthe belt; two electromagnetic switches electrically connectedrespectively to the contacting switches and mounted on the baserespectively at two sides of the driven axle bracket; and two springseach having two ends connected respectively one of the electromagneticswitches and the driven axle bracket, wherein the springs are connectedrespectively to the two sides of the driven axle bracket to providepulling forces in opposite directions to the driven axle bracket. 12.The wide belt sander as claimed in claim 11, wherein each contactingswitch has a contacting arm protruding from the contacting switch,selectively abutting with the belt and having a ceramic sleeve mountedaround the contacting arm.
 13. The wide belt sander as claimed in claim1, wherein the automatic adjustment device comprises a gear boxadjustably mounted on the base; a driving shaft rotatably mounted on andextending into the gear box; a driven shaft rotatably mounted on andextending out from the gear box and to which the cam is attached; and agear device mounted in the gear box, mounted between the driving shaftand the driven shaft and comprising a worm rod co-axially mounted on thedriving shaft; and a worm gear securely mounted on the driven shaft andengaging the worm rod.
 14. The wide belt sander as claimed in claim 13,wherein the pushed elements are rotatably mounted on a bottom of thedriven axle bracket at an interval and have a center line definedbetween centers of the pushed elements; and the cam has a rotatingcenter where is mounted around the driven shaft and located at thecenter line between the pushed elements.
 15. The wide belt sander asclaimed in claim 14, wherein each pushed element is a bearing having acentral hole defined through the pushed element; and two securing boltsmounted respectively through the central holes of the pushed elementsand mounted securely to the bottom of the driven axle bracket.
 16. Thewide belt sander as claimed in claim 13, wherein the base has a channeldefined along a direction perpendicular to the central axis of thedriving axle; and two threaded holes defined in the base adjacent to thechannel; and the gear box is mounted adjustably in the channel of thebase and has a bottom; two ribs formed on the bottom and slidably heldin the channel in the base; a wing mounted on and laterally protrudingfrom the bottom of the gear box and having two elongated holes definedthrough the wing and aligning respectively with the threaded holes inthe base; and two bolts mounted respectively through the elongated holesand screwed respectively into the threaded holes in the base.
 17. Thewide belt sander as claimed in claim 1, wherein the base furthercomprises a holding bracket to which the driven axle bracket ispivotally connected and the gear box is adjustably mounted and having anaxle hole defined in a top of the holding bracket; the driven axlebracket has a central shaft formed on and protruding downward from thedriven axle bracket and rotatably and slidably extending into the axlehole in the holding bracket; and a pneumatic cylinder is mounted betweenthe holding bracket and the driven axle bracket and has a first endprovided with a universal jointer and connected to the holding bracketwith the universal jointer; and a second end connected co-axially to thecentral shaft on the driven axle bracket to provide a supporting effectto the driven axle bracket.
 18. The wide belt sander as claimed in claim17 further comprising a limiting device mounted between the driven axlebracket and the holding bracket to limit the swinging range of thedriven axle bracket and comprising a limiting bracket mounted on thebottom of the driven axel bracket, being inversed U-shaped and havingtwo ears respectively formed on and protruding from two ends of thelimiting bracket; two limiting bolts mounted respectively on the ears ofthe limiting bracket and protruding toward each other; and a basebracket mounted securely on the top of the bolding bracket and having atop extending into the limiting bracket to a position between thelimiting bolts.